Treatment of Sendai virus and NDV nucleocapsids with S. aureus protease cleaves polypeptide P into two domains, an acidic, protease-sensitive portion, and a basic, protease-resistant core, but has little to no effect on polypeptides NP and L. Cleavage of NDV polypeptide P resulted in a proportional loss of RNA synthetic activity suggesting that P is involved in the synthesis of viral RNA. Three approaches were used to show that the hemagglutinating and neuraminidase activities of the HN glycoprotein of paramyxoviruses involve two distinct sites. These activities were separated by analyzing a Sendai virus mutant with a ts HN protein and the use of monoclonal antibodies to HN and an analog of N-acetyl-neuraminic acid. The mRNAs of Sendai virus were separated on agarose urea gels and the RNAs responsible for synthesis of P, NP, and M proteins identified. The ts HN protein of mutant ts 271 was shown to be rapidly degraded after synthesis.